In recent years, the distance and capacity of an optical communication network have been increasing with a growing demand for a communication made by a broadband service. For example, high-speed and large-capacity WDM (Wavelength Division Multiplexing for simultaneously transmitting a plurality of signals with a single optical fiber by multiplexing light beams of different wavelengths) is currently under development.
In the meantime, a higher-speed, larger-capacity and flexible optical communication network has been demanded with the rapid popularization of the Internet, and an increase in large capacity content. An optical packet switching technique draws attention as a technique for building such an optical communication network.
The optical packet switching is a technique for making packet switching by using communication information totally unchanged as light. This technique eliminates restrictions on an electronic processing speed in comparison with conventional switching for once converting an optical signal into an electric signal. Therefore, the processing speed depends on only the propagation delay time of light, and accordingly, a high-speed and large-capacity transmission can be made.
If an optical signal is switched in units of packets, gate switches are used to turn on/off the optical signal. The gate switches for turning on/off an optical signal with an electric control mainly include an electro-absorption gate switch and a semiconductor optical amplifier gate switch. The electro-absorption gate switch is intended to change optical absorption by using electro-absorption effect. However, this gate switch has a disadvantage of a large loss even in a transmission state. In contrast, the semiconductor optical amplifier gate switch is intended to change a gain with a driving current applied to a semiconductor amplifier, and has not only a function as an optical gate for turning light on/off but also an amplification function (to amplify and output light when a gate is turned on). Accordingly, this gate switch currently attracts attention as an optical element that reduces a loss of an optical signal and makes high-speed switching.
For an SOA, its extinction ratio of ON (open) to OFF (closed) of a gate is high, and its amplification mechanism can reduce an optical loss. Since the SOA is an optical element formed with a semiconductor, it has an advantage of downsizing enabled at low cost with a semiconductor integration technique. (The extinction ratio is a ratio of the average light intensity of signals “1” and “0” when a gate is ON to that of signals “1” and “0” when the gate is OFF. As the extinction ratio becomes higher, ON/OFF of a gate can be more explicitly identified. As a result, signal crosstalk that affects another port can be reduced, and a bit error rate becomes low.)
In the meantime, reductions in the size, the consumed power and the cost of a module or a device by collectively forming a plurality of channels, namely, by arraying a plurality of channels have been demanded for an optical gate switch.                Japanese Laid-open Patent publication No. 2007-33853.        Japanese Laid-open Patent publication No. 2008-235376.        Farries, M. C. Buus, J. Robbins, D. J. “Analysis of antireflection coatings on angled facet semiconductor laser amplifiers,” Electronics Letters, 15 Mar. 1990, Volume: 26, pp. 381-382.        
Normally, when optical coupling between an optical element that outputs light and an optical fiber is made, the optical element and a first lens are optically coupled at first and output light from the optical element is converted into parallel light by the first lens. Then, there is used a configuration in which its parallel light is condensed by a second lens to be made incident on the optical fiber (Also, there is used a configuration in which parallel light is generated to be condensed into the optical fiber only by the first lens).
FIG. 23 illustrates optical coupling between a laser diode (LD) and a lens. FIG. 23 illustrates the optical coupling at the time of using a single channel LD as the optical element. In addition, in a subsequent description, the optical element and the first lens are illustrated, and the second lens that condenses parallel light and the optical fiber are omitted.
FIG. 23 illustrates a configuration in which a central point (central line) of a spread angle of output light from the LD 51 is caused to coincide with a central line of the lens 52 and the output light from the LD 51 is converted into parallel light by the lens 52.
FIG. 24 illustrates the optical coupling between the LD array and the lens array. FIG. 24 illustrates a case of four channels as the optical coupling of multi-channel. An LD array 51a is one obtained by arraying four LDs and a lens array 52a is one obtained by arraying four lenses.
Similarly to FIG. 23, the central point of a spread angle of output light from each LD of the LD array 51a is caused to coincide with the central line of a corresponding lens of the lens array 52a, thereby outputting parallel light from the lens array 52a. The LD array 51a and the lens array 52a have the same array pitch.
Next, consider a case where the optical coupling is made by using the SOA in place of the LD. Here, for an optical gate switch using an SOA, a reflectance of the light output end face of the SOA must be reduced in order to prevent an oscillation caused by the internal reflection of the SOA.
For this purpose, an Anti Reflection (AR) coat as a non-reflective film is normally applied to an end face of an SOA chip. Further, since a reflection attenuation amount is not sufficiently reduced only based on the AR coat, there is further adopted a configuration in which reflected return light is suppressed by using a shape in which an intra-SOA waveguide is inclined.
FIG. 25 illustrates an SOA array. FIG. 25 illustrates an SOA array 61a obtained by arraying the SOA of four channels. An AR coat is applied to each end face of the SOA of the SOA array 61a. Further, an intra-SOA waveguide L is obliquely formed such that light is output obliquely, for example, by 22.3 degrees with respect to the normal to the end face of the SOA array.
By adopting the above-described structure, even if light passing through the intra-SOA waveguide L is reflected on a chip end face of the SOA, its reflected light goes back in the direction A illustrated in FIG. 25. For this reason, the reflected light is prevented from returning again to the intra-SOA waveguide L and from causing interference, thus suppressing reflected return light. While FIG. 25 illustrates only the central point (central line) of a spread angle of the output light beam (light beam spread angle), a light beam output from the SOA of each channel has a certain spread angle.
Next, optical coupling between the SOA and the lens will be described. FIG. 26 illustrates the optical coupling between the SOA and the lens. FIG. 26 illustrates the optical coupling of a single-channel SOA. When making optical coupling between the SOA 61 and the lens 62, the central point of the light beam spread angle of the SOA 61 is inclined obliquely as described above. Accordingly, as illustrated in FIG. 26, a central line of output light from the SOA 61 is made incident obliquely to a central position p0 of the lens 62 in a state where an end face of the SOA 61 and a main face of the lens 62 are arranged in parallel.
FIG. 27 illustrates optical coupling between the SOA array and the lens array. The optical module 6 makes optical coupling of a multi-channel SOA module. Similarly to FIG. 26, light that is output obliquely by 22.3 degrees with respect to an end face of the SOA is made incident on the lens array 62a with the same array pitch as that of the SOA array 61a, thus making the optical coupling.
In configurations illustrated in FIGS. 26 and 27, (disclosed in Japanese Laid-open Patent publication No. 2007-33853 as described above), output light from the SOA array 61a is incident obliquely to the central point of each lens of the lens array 62a (central beam of output light from each SOA of the SOA array 61a and a central line of a corresponding lens of the lens array 62a are not parallel to each other). Therefore, among beams after passing through the lens array 62a, a central beam b0 in the beam spread angle of the SOA and two arbitrary refracted beams b1 and b2 other than the central beam b0 do not become parallel. For the above-described reason, there is a problem that parallel beams are not generated in the lens array 62a and an optical coupling loss increases with respect to the optical fiber array arranged in a subsequent stage (not illustrated).
To cope with the above-described problem, in a conventional technique disclosed in Japanese Laid-open Patent publication No. 2008-235376 as described above, the optical coupling is made by causing a central line of the beam spread angle of the SOA to coincide with that of the lens in the same manner as in a case of the LD.
FIG. 28 illustrates optical coupling between the SOA and the lens. In the optical module 6, a central beam of output light from the SOA 61 coincides with a central line of the lens 62 in parallel. When the SOA 61 and the lens 62 are arranged as described above, the central beam in the beam spread angle of the SOA 61 and the refracted side beams go to the same direction. Therefore, the parallel beams from the lens 62 are output and an increase in the optical coupling loss can be suppressed.
FIG. 29 illustrates optical coupling between the SOA array and the lens array. FIG. 29 illustrates a state where the SOA array 61-1 provided with steps and the lens array 62a are arranged in the optical module 6-1.
To enable the central beam of output light from the SOA 61 to coincide with the central line of the lens 62, as illustrated in FIG. 28, and to enable channels to have the same distance between the SOA end face and the lens main face, the SOA array 61-1 having the steps in the array of the SOA chips, as illustrated in FIG. 29, is manufactured.
FIG. 30 illustrates a configuration in which optical coupling between the SOA array and the lens array is made. FIG. 30 illustrates a state where the SOA array 61a and the lens array 62-1 provided with steps are arranged in the optical module 6-2.
To enable the central beam of output light from the SOA 61 to coincide with the central line of the lens 62, as illustrated in FIG. 28, and to enable the channels to have the same distance between the SOA end face and the lens main face, the SOA array 62-1 having the step in the array of the lens, as illustrated in FIG. 30, is manufactured.
However, also in both of the configurations of FIGS. 29 and 30, the SOA array 61-1 and the lens array 62-1 have distorted shapes, and the SOA array 61-1 and the lens array 62-1 on which the above-described steps are provided are difficult to manufacture. As a result, a component manufacturing cost of the SOA array 61-1 and the lens array 62-1 largely increases.
As described above, in the conventional structure (optical module 6) illustrated in FIG. 27, since parallel light is not generated, the optical coupling loss increases with respect to the optical fiber arranged at a subsequent stage. In the conventional structures (optical modules 6-1 and 6-2) illustrated in FIGS. 29 and 30, parallel light is generated; however, manufacturing the SOA array or lens array with a complicated shape costs higher.